Pins made from graphite, resin and titanium have been used in the fabrication or composite structures comprising a foam core and facesheets. The purpose of the pins is to enhance the peel strength (interlaminar strength) between the composite layers and between the facesheet and the foam core in the case of sandwich structures. U.S. Pat. No. 5,736,222 (Childress), U.S. Pat. No. 4,808,461 (Boyce et al.), U.S. Pat. No. 5,980,665 (Childress) and U.S. Pat. No. 5,935,475 (Scoles and Woodley) describe techniques for the insertion of pins into composite structures. These patents all use low-density degradable polymer foams as pin carriers to install pins into B-staged composites during autoclaving at elevated temperatures and pressures. During autoclaving, the foam decomposes and the pins are pushed into the B-staged composite by external pressure acting on the caul plate. Low-density polymethacrylic imide foams (ROHACELL™) and polystyrene (STYROFOAM™) are used in these applications. Thus, in these prior art processes, the polymer foam is used as a pin carrier and sacrificial medium, as opposed to a structural element that remains permanently in the composite foam after fabrication as in the case of the instantly described and claimed process.
Other patents like U.S. Pat. No. 5,186,776 (Boyce et al.), U.S. Pat. No. 7,387,147 B2 (Johnson et al.) and U.S. Pat. No. 7,105,071 B2 (Johnson et al.) describe techniques for inserting Z-pins into fiber reinforced polymeric composites. To avoid damage to the fiber in the composite these patents utilize techniques that soften the composite allowing the pins to deflect the fiber when they penetrate the composite. Such softening can negatively affect the properties of the composite
Structural foams useful in composite or sandwich structures are currently processed from polymers like polymethacrylic imide and polyvinyl urea-amides. Other commonly used polymer foams include polyurethanes, polyimides, polyethylene, polycarbonate, phenolics, elastomers, and acrylics. Thermotropic liquid crystalline polymers (LCPs) have been extensively studied in recent years for electronic applications because of their high thermal, mechanical and chemical properties. Specifically useful foams and their methods of manufacture are described in greater detail in commonly assigned U.S. Pat. No. 6,232,354 and U.S. patent application Ser. No. 11/807,488, both of which are incorporated herein by reference in their entirety.
Thus, while structurally desirable foams and their methods of manufacture as well as pin or tube reinforced composites have been described in the prior art the marriage of these two technologies to provide superior structural composites and methods for their manufacture has neither been described nor suggested.